Surface Water - Groundwater Interactions and Their Control on Groundwater Redox Processes

University of Minnesota M.S. thesis. February 2020. Major: Earth Sciences. Advisor: Gene-Hua Ng. 1 computer file (PDF); viii, 118 pages.Interactions between surface water and groundwater promote sharp geochemical and redox gradients, diverse microbial metabolisms, and the transformation of contaminants. Due to the impact on water quality, it is important to develop a greater understanding of surface water-groundwater interaction and hyporheic zone systems. Here, two projects are discussed, both focus on investigating surface water-groundwater interactions and their impacts on biogeochemical cycles occurring in the subsurface. In chapter one, a reactive transport model was developed to simulate seasonal temperature variability in an aquifer downgradient from a nutrient impacted lake. Model results indicate that As release is temperature dependent and implementing temperature variation results in As concentrations above maximum contaminant levels (MCL). Above MCL As concentrations were unable to be reproduced in scenarios where temperature was simplified to an averaged value. Chapter two outlines field and lab-based methods used to observe hydrologic, geochemical, and microbial drivers of “cryptic” sulfur cycling within a heterogeneous and diverse riparian wetland of the Savannah River in Aiken, SC. Results indicate that dynamic hydrologic conditions have a marked impact on subsurface geochemistry. Subsites which experienced periods of inundation and non-inundation had higher dissolved and solid Fe concentrations and elevated sulfate concentrations. Additionally, hydrologic flux direction appears to be linked to the production of Fe oxide microbial mats. Sites which experienced upward flux of groundwater contained flocs, whereas sites which experienced downward flux did not. Finally, sites containing microbial mats were found to contain S intermediates, indicating the presence of “cryptic” sulfur cycling